1. Field of the Invention
The present invention relates to plasma processing systems and, more particularly, to methods and apparatus for controlling radio frequency delivery in a plasma reactor through monitoring and feedback of an electrical parameter, in particular a peak voltage.
2. Background Art
Ionized gas, or plasma, is commonly used during the processing and fabrication of semiconductor devices. For example, plasma can be used to etch or remove material from semiconductor integrated circuit wafers, and to sputter or deposit material onto semiconducting, conducting or insulating surfaces.
With reference to FIG. 1A, creating a plasma for use in manufacturing or fabrication processes typically begins by introducing various process gases into a plasma chamber 10 of a plasma reactor, generally designated 12. These gases enter the chamber 10 through an inlet 13 and exit through an outlet 15. A workpiece 14, such as an integrated circuit wafer is disposed in the chamber 10 held upon a chuck 16. The reactor 12 also includes plasma density production mechanism 18 (e.g. a TCP coil). A plasma inducing signal, supplied by a plasma inducing power supply 20 is applied to the plasma density production mechanism 18. the plasma inducing signal is preferably a radio frequency (RF) signal. A dielectric window 22, constructed of a material such as ceramic, incorporated into the upper surface of the chamber 10 allows efficient transmission of the first RF signal from the TCP coil 18 to the interior of the grounded chamber 10. This first RF signal excites the gas molecules within the chamber, generating a plasma 24.
The plasma 24 formed within the chamber 10 includes electrons and positively charged particles. The electrons, being lighter than the positively charged particles tend to migrate more readily, causing a sheath to form at the surfaces of the chamber 10. A self biasing effect causes a net negative charge at the inner surfaces of the chamber. This net negative charge, or D.C. sheath potential acts to attract the heavier positively charged particles toward the wall surfaces. The strength of this D.C. bias in the location of the workpiece 14 largely determines the energy with which the positively charged particles will strike the workpiece 14 and correspondingly affects the desired process (e.g. etch rate, or deposition rate).
The present invention will be more readily understood by bearing in mind the distinction between DC bias and DC sheath potential. DC bias is defined as the difference in electrical potential between a surface within the chamber 10 and ground. DC sheath, on the other hand is defined as the difference between the plasma potential and the potential of a surface within the chamber as measured across the plasma sheath.
The workpiece is held upon a chuck 16 is located at the bottom of the chamber 10 and constitutes a chuck electrode 26. A bias RF power source 28 supplies a biasing RF signal to the chuck electrode 16. Alternatively, in some systems the both the plasma density signal and bias signal are in fact a single signal produced by a single power source.
This second excitation signal, preferably in the form of a RF signal, at the second electrode increases the DC bias at the location of the workpiece, depending on the disposition of the RF electric field within the chamber 10, and this increases the energy with which the charged particles strike the workpiece. Variations in the RF signal supplied to the second electrode 16 produce corresponding variations in the D.C. bias at the workpiece affecting the process.
With continued reference to FIG. 1A, the bias RF power source 28 described above supplies a R.F. signal to the chuck electrode 26. This signal passes through a match network 30 disposed between the bias RF power source 28 and the chuck electrode 26. The match network 30 matches the impedance of the RF signal with the load exhibited by the plasma. A similar match network 31 is provided between the power inducing power source 20 and the TCP coil 18. As discussed above, the control and delivery of the RF signal at the chuck electrode 26 is of fundamental importance in plasma processing. Significant variance in actual power delivered may unexpectedly change the rate of the process. Unfortunately, the match network 30 generates significant losses in the RF signal. Furthermore, these losses are variable and, to a degree, unpredictable. Therefore, simply supplying a predetermined RF signal power from the RF power source 28 does not ensure that a predictable and consistent RF signal will be delivered at the electrode 26.
With continued reference to FIG. 1A, one method which has been used to attach the workpiece 14 to the chuck 16 has been to provide the chuck with clamps 32 which contact the surface of the workpiece along its edges to hold the workpiece to the chuck. Using such a chuck 16 (and to the extent that the workpiece is somewhat conductive) it is possible to measure the D.C. bias directly by installing a pickup 33 at the electrode 26 and transmitting a voltage signal to a voltage sensor 34. The power source could then be feedback controlled to maintain a constant measured D.C. bias. However, using such clamps 32 to attach the workpiece 14 to the chuck 16 presents multiple problems. For one, valuable surface area may be wasted on the workpiece due to its engagement with the clamps 32. In addition, any such contact of clamps 32 to the workpiece 14 is undesirable due the risk of damage to the workpiece 14, and the generation of particles.
With reference to FIG. 1B, another method which has been used to hold the workpiece onto the electrode has been to provide an electrode in the form of an electrostatic chuck 36. In its most general sense an electrostatic chuck includes an electrode 38 which is covered with an insulator 40. The electrically conductive workpiece 14, which is generally semiconductive, sits on the electrically insulating material. When a DC voltage is applied to the electrode 38, the electrode and workpiece 14 become capacitively coupled resulting in opposite electrical charges on each, attracting the workpiece 14 and electrode 38 toward one another. This acts to hold the workpiece against the chuck 36.
More particularly, the electrostatic chuck 36 can be understood with reference to FIG. 1C in addition to FIG. 1B. In this bipolar implementation, the electrode 38 of the electrostatic chuck 36 includes first and second electrically conducive portions 42 and 44, which are electrically isolated from one another. A DC voltage from a D.C. voltage source 46, passes through a filter 47 before being applied between the first and second portions 42 and 44 of the electrode 38. This causes the desired electrostatic attraction between the electrode 38 and the workpiece 14, thereby holding the workpiece to the chuck 36.
With reference to FIG. 1D, a simpler version of electrostatic chuck is illustrated. This simpler form of electrostatic chuck, termed a mono polar chuck 37 is shown in plan view in 1D. By applying a DC potential between the workpiece 14 and the chuck an electrostatic charge on each holds the workpiece to the chuck. It will be appreciated by those skilled in the art that numerous other forms of electrostatic chuck are possible as well.
However, use of such an electrostatic chuck 36 renders a direct measurement of the D.C. bias at the workpiece impractical. End users are averse to having their sensitive semiconductor products touched by any mechanical probe or electrically conductive item such as a voltage sensor. In addition, it would be difficult to maintain sensor accuracy and longevity in the plasma environment. Correlating the D.C. voltage by measuring the power of the RF signal at the electrode 16 is also difficult and does not provide an accurate measurement of the D.C. sheath potential due, in part, to the capacitive coupling between the electrode and the workpiece.
Therefore, there remains a need for system for controlling R.F. power at an electrode to maintain a consistent D.C. sheath potential. Such a system would preferably not involve contact with a workpiece, would not require placing a sensor with the plasma environment of the plasma chamber, and would account for variable and unpredictable power losses through a match network.
The present invention provides a plasma reactor having a chamber and a chuck supporting a workpiece within the chamber. The chuck includes a chuck electrode which receives a bias radio frequency (RF) signal from a bias RF power source. The RF signal at the electrode affects the plasma, and more particularly affects the DC bias. A sensor measures a parameter of the plasma, such as for example the peak voltage of the RF signal delivered to the electrode which is compared with the desired set point and from which an error signal is derived. The error signal is then amplified and used to control the RF power source.
Typically, a match network, located between the bias RF power source and the chuck, matches the impedance of the plasma load to that of the output (typically 50 Ohms) of the RF power source. The maintenance of a consistent RF signal at the electrode is of importance in maintaining a consistent DC bias at the workpiece and a correspondingly consistent process. For instance the RF delivery system is subject to losses such that process results may not be predictable and constant. For example, the match network generates substantial power losses in the RF signal, these losses being variable and, to an extent, unpredictable. By sensing the RF peak voltage near the electrode and using that sensed voltage to generate a corresponding error signal to control the power supply, a consistent D.C. bias can be maintained at the workpiece in spite of the variation in transmission, such as for example those generated by the match network.
More particularly, the present invention is preferably embodied in an inductive plasma reactor having a Transformer Coupled Plasma (TCP) reactor coil. This coil can be located outside of the plasma chamber and is separated from the plasma by a ceramic window, provided in wall of the chamber. A plasma generating RF source supplies a RF signal to the TCP coil. A gas flows through the chamber, and is ionized by RF current induced from the TCP coil. The RF current is coupled to the plasma primarily by magnetic induction through a dielectric window. The fundamental purpose of the TCP coil and the signal supplied thereto is to generate plasma density.
As the plasma is formed, the electrons, which tend to migrate more easily than the positive ions, develop a net negative charge on the at the inner surfaces of the chamber as well as at the workpiece supported upon the chuck. This net charge generates a DC bias which determines the energy with which the positively charged particles strike the surface of the workpiece and thereby is a primary factor in determining the process results.
A pickup connected to the electrode receives the RF signal delivered to the electrode. This signal is then passed through a lead wire to the RF sensor which is located as close to the chuck electrode as is possible without risking arcing between the sensor and the chuck electrode. Placing the sensor close to the electrod minimizes the length of lead wire necessary to transmit the RF signal to the sensor, thereby minimizing inductive and resistive affects of the lead wire upon the signal.
Within the sensor, the RF signal is divided and separated into AC and DC components. If desired, the DC component can be used to monitor electrostatic chuck function. However that is not a necessary component of the present invention. The AC signal component then passes through a surge protection circuit before being fed to a balanced detector circuit. The balanced circuit ensures that the AC signal is symmetrically loaded about the zero volt axis, ensuring that the signal does not generate a spurious DC component which would induce error into the system. The AC signal is then passed through an amplifier circuit which includes a feedback circuit and incorporating retification and peak hold circuitry to yield a DC equivalent of the RF peak voltage. Matched diodes in both arms of the amplifier circuit, together with the diode in the balance circuit, ensure that any non-linearity is largely compensated or in the DC equivalent signal at the output of the amplifier.
This DC equivalent signal is then passed through a differential buffer and an amplifier with gain and offset adjustment before being delivered as an output signal. This same signal is then compared with the desired setpoint to derive an error signal which is passed through a high gain amp and through a power limit circuit which protects the electrode from being damaged by a surge. The signal passes to the generator to provide a RF generator as a command to control the power produced.
Alternatively the present invention can be used with a capacitively coupled plasma reactor. In such an electrode capacitively coupled with the chuck electrode replaces the TCP coil described above. In addition, the present invention can be used with a mechanical chuck rather than an electrostatic chuck obviating the need to place a sensor within the plasma environment.
By detecting the RF peak voltage delivered at the electrode, the present invention accurately and efficiently controls the RF signal delivered to the chuck elecrode, allowing a consistent DC bias to be maintained. In this way, the plasma reactor can consistently produce high quality uniform workpieces.
While the invention has been described in terms of using RF peak voltages delivered to the electrode, it should be appreciated that other process parameters can be monitored as well and used in a feedback system to control the process. By way of example, the current supplied to the coil could be monitored and used in a feedback system.
These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following descriptions of the invention and a study of the several figures of the drawings.